High power handling polarization switches

ABSTRACT

Liquid crystal devices are described that maintain performance of polarization/amplitude modulation under high irradiance conditions. Configurations that isolate polarizing elements under high thermal load are discussed which allow other elements, such as glass, which may be sensitive to stress birefringence to remain near optimum thermal conditions.

TECHNICAL FIELD

This disclosure generally relates to displays, and more specificallyrelates to stereoscopic flat panel displays having a liquid crystal (LC)modulation panel, and a polarization control panel (PCP).

BACKGROUND

Polarization switches are frequently used to temporally encodestereoscopic imagery for single-projector 3D projection display. Anexample is the ZScreen™, which may include a neutral linear inputpolarizer, followed by alternately engaged liquid crystal pi-cells. Asthe output from many projector models is substantially unpolarized, suchas those based on the Texas Instruments DLP microdisplay, more than halfof the energy is absorbed by the input polarizer of the polarizationswitch. This energy is dissipated in the optical assembly, resulting inlocalized heating. The polarization switch is usually assembled usingoptical adhesives to eliminated air-glass interfaces that produce lightloss and degradation in performance (e.g. contrast and transmittedwavefront distortion). Localized heating in such an assembly causes adistribution in strain, usually resulting in significant birefringencethat degrades performance. In 3D display, this is manifested as a lossin the stereo contrast ratio (SCR).

BRIEF SUMMARY

According to an aspect of the present disclosure, a polarization switchmay include a first assembly operable to receive light. The firstassembly may include a first end cap, a first polarizer located toreceive light from the first end cap, a second end cap located toreceive light from the first polarizer. The second assembly may belocated to receive light from the first assembly, and may include athird end cap, a first cell located to receive light from the third endcap, a second cell located to receive light from the first cell, afourth end cap located to receive light from the second cell, wherein athermal isolation structure separates the first assembly from the secondassembly. The thermal isolation structure may be in the approximaterange of 0.5-10 mm. In some embodiments, the thermal isolation structuremay be an air gap.

Further, the first end cap may be borofloat glass and may be coated withan anti-reflective coating. The thickness of the first, second, thirdand fourth end caps may be in the approximate range of 3-12 mm. Thesecond end cap may be synthetic fused silica. The second assembly mayfurther include a second polarizer. The first polarizer may be an iodineor a dye polarizer and the second polarizer may be a dye or an iodinepolarizer. The first cell and the second cell may be liquid crystalcells, for example pi-cells. The second assembly may further include afirst compensator located to receive light from the first cell, a secondcompensator located to receive light from the first compensator, and athird compensator located to receive light from the second cell. Thefirst, second, and third compensators may be -C compensators.

According to another aspect of the present disclosure, a high powerhandling polarization system may include a first optical assemblyoperable to receive light. The first optical assembly may include afirst substrate operable to receive light, a first polarizer adjacent tothe first substrate and operable to receive light from the firstsubstrate, and a planarization layer adjacent to the first polarizer.The high power handling polarization system may also include a secondoptical assembly operable to receive light from the first opticalassembly. The second optical assembly may include a second substratelocated to receive light from the planarization layer of the firstoptical assembly, a first liquid crystal cell located subsequent in thelight path to the second substrate, a second liquid crystal cell locatedto receive light from the first liquid crystal cell, a third substratelocated subsequent in the light path to the second liquid crystal celland a thermal isolation structure may be located between the firstoptical assembly and the second optical assembly. The thickness of thethermal isolation structure may be in the approximate range of 0.5-10mm. In some embodiments, the thermal isolation structure may be an airgap. The second optical assembly may also include a second polarizerlocated to receive light from the second substrate, wherein the secondpolarizer may be a dye or an iodine polarizer and the first polarizer inthe first optical assembly may be a dye or an iodine polarizer. Further,the first and second polarizers may be dye polarizers, iodinepolarizers, or any combination thereof.

Additionally, the first second and third end caps may be borofloat glassand the first and second liquid crystal cells may be pi-cells. Theplanarization layer may be coated with an anti-reflective coating andmay be approximately index-matched to the first polarizer substrate.Also, the second optical assembly may include a first and secondcompensator located between the first and second liquid crystal cellsand a third compensator located after the second liquid crystal cell,and before the third end cap, wherein the first, second, and thirdcompensators may be -C compensators, for example Zeon 250.

According to another aspect of the present disclosure, liquid crystaldevices are described that maintain performance ofpolarization/amplitude modulation under high irradiance conditions.Configurations that isolate polarizing elements under high thermal loadare discussed which allow other elements, for example, glass, that aresensitive to stress birefringence to remain near optimum thermalconditions.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1 is a schematic diagram illustrating the loss in stereo contrastratio with lumens incident on a cinema polarization switch, inaccordance with the present disclosure;

FIGS. 2A and 2B are schematic diagrams illustrating two configurationsfor high power handling polarization switches, in accordance with thepresent disclosure;

FIG. 3 is a schematic diagram illustrating stability of a high powerhandling polarization switch under the lumen loading of FIG. 1, inaccordance with the present disclosure;

FIGS. 4A, 4B, and 4C are schematic diagrams illustrating optical systemsthat employ the high power handling polarization switches; and

FIGS. 5A and 5B are schematic diagrams illustrating two configurationsfor high power handling polarization switches, in accordance with thepresent disclosure.

DETAILED DESCRIPTION

Polarization switches are frequently used to temporally encodestereoscopic imagery for single-projector 3D projection display. Anexample is the ZScreen™, which may include a neutral linear inputpolarizer, followed by alternately engaged liquid crystal pi-cells. Asthe output from many projector models is substantially unpolarized, suchas those based on the Texas Instruments DLP microdisplay, more than halfof the energy is absorbed by the input polarizer of the polarizationswitch. This energy is dissipated in the optical assembly, resulting inlocalized heating. The polarization switch is usually assembled usingoptical adhesives to eliminated air-glass interfaces that produce lightloss and degradation in performance, for example, contrast andtransmitted wavefront distortion. Localized heating in such an assemblycauses a distribution in strain, usually resulting in significantbirefringence that degrades performance. In 3D display, this ismanifested as a loss in the stereo contrast ratio (SCR). The SCR is theratio of luminance of the intended image transmitted through the 3Deyewear lens, to that intended for the other eye. Optical assembliesthat optimize SCR and other optical performance characteristics can beproduced using embodiments of the present disclosure.

According to an aspect of the present disclosure, a polarization switchmay include a first assembly operable to receive light. The firstassembly may include a first end cap, a first polarizer located toreceive light from the first end cap, a second end cap located toreceive light from the first polarizer. A second assembly may be locatedto receive light from the first assembly, and may include a third endcap, a first cell located to receive light from the third end cap, asecond cell located to receive light from the first cell, a fourth endcap located to receive light from the second cell, wherein a thermalisolation structure separates the first assembly from the secondassembly. The thermal isolation structure may be in the approximaterange of 0.5-10 mm.

Further, the first end cap may be borofloat glass and may be coated withan anti-reflective coating. The thickness of the first, second, thirdand fourth end caps may be in the approximate range of 3-12 mm. Thesecond end cap may be synthetic fused silica. The second assembly mayfurther comprise a second polarizer located to receive light from thethird end cap. The first polarizer may be an iodine or a dye polarizerand the second polarizer may be a dye or an iodine polarizer. The firstcell and the second cell may be liquid crystal cells, for examplepi-cells. The second assembly may further comprise a first compensatorlocated to receive light from the first cell, a second compensatorlocated to receive light from the first compensator, and a thirdcompensator located to receive light from the second cell. The first,second, and third compensators may be -C compensators.

According to another aspect of the present disclosure, a high powerhandling polarization system may include a first optical assemblyoperable to receive light. The first optical assembly may include afirst substrate operable to receive light, a first polarizer adjacent tothe first substrate and operable to receive light from the firstsubstrate, and a planarization layer adjacent to the first polarizer.The high power handling polarization system may also include a secondoptical assembly operable to receive light from the first opticalassembly. The second optical assembly may include a second substratelocated to receive light from the planarization layer of the firstoptical assembly, a first liquid crystal cell located subsequent in thelight path to the second substrate, a second liquid crystal cell locatedto receive light from the first liquid crystal cell, a third substratelocated subsequent in the light path to the second liquid crystal celland a thermal isolation structure located between the first opticalassembly and the second optical assembly. The thickness of the thermalisolation structure may be in the approximate range of 0.5-10 mm. Thesecond optical assembly may also include a second polarizer located toreceive light from the second substrate, wherein the second polarizermay be a dye or an iodine polarizer and the first polarizer in the firstoptical assembly may be a dye or an iodine polarizer. Further, the firstand second polarizers may be dye polarizers, iodine polarizers, or anycombination thereof.

Additionally, the first second and third end caps may be borofloat glassand the first and second liquid crystal cells may be pi-cells. Theplanarization layer may be coated with an anti-reflective coating andmay be approximately index-matched to the first polarizer substrate.Also, the second optical assembly may include a first and secondcompensator located between the first and second liquid crystal cellsand a third compensator located after the second liquid crystal cell,and before the third end cap, wherein the first, second, and thirdcompensators may be -C compensators, for example Zeon 250.

According to another aspect of the present disclosure, liquid crystaldevices are described that maintain performance ofpolarization/amplitude modulation under high irradiance conditions.Configurations that isolate polarizing elements under high thermal loadare discussed which allow other elements, for example, glass, that aresensitive to stress birefringence to remain near optimum thermalconditions.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

FIG. 1 is a schematic diagram illustrating the loss in stereo contrastratio with lumens incident on a cinema polarization switch. In FIG. 1,the cinema polarization switch may be a theatrical ZScreen™. Zscreensare generally discussed at least in commonly owned U.S. Pat. No.4,792,850 and U.S. Pat. No. 7,477,206 both of which are hereinincorporated by reference in their entireties. FIG. 1 shows the impactof localized heating of a 3D polarization switch when used with aDLP-based digital cinema projector. As the lumen output is increased, sotoo is the thermal loading on the optical assembly. The associatedstrain produces stress-birefringence in the glass that degrades theprecise polarization control needed to maintain SCR. Stressbirefringence in any glass subsequent to the input polarizer contributesto this degradation. This includes the liquid crystal cells, and anyexit end-cap needed for optical performance. The figure shows thatsignificant loss in SCR occurs for typical lumen outputs needed toachieve brightness standards on average sized cinema screens. Whencombined with SCR losses associated with other elements of the system,the result is ghosting that is beyond acceptable levels. Research hasshown that reduced SCR produces loss in perceived image depth, and evenvisual discomfort.

Sheet polarizer is typically composed of a functional PVA layer boundedby TAC protective films. The polarizer is frequently bonded to one glasslayer, for example, the input glass endcap, using a pressure-sensitiveadhesive, with the other surface of the polarizer buried in the opticalassembly using a thermoset or UV cure adhesive. The refractive indexesof the TAC, glass, and adhesives are nearly matched, so the transmittedwavefront distortion of the assembly can be quite good.

Conventional sheet polarizer is fabricated using web-based processes,which make it difficult to achieve a low transmitted wavefrontdistortion (TWD). Undulations in the local optical path-length throughthe structure cause TWD, which is easily observed in a free-standingfilm measurement. Moreover, irregularity can be observed in reflection(single-side bonded) due to the surface profile, with larger scaleflatness issues frequently introduced from the lamination process.However, when both surfaces are index matched between optically-flatglass, these issues are reduced to an acceptable level.

In most instances, dichroic sheet polarizer used in thermally demandingsituations requires more durable dye-stuff chemistry, versus more commoniodine chemistry. Dye-type polarizers can withstand higher temperatures,though their efficiencies, such as internal transmission efficiency andpolarizing efficiency, tend to be lower than iodine polarizer. Reducingpolarizing efficiency also reduces the stereo contrast ratio, while lossin internal transmission degrades the 3D image brightness. Frequently,dye polarizers suffer lower transmission in the blue portion of thespectrum, so system losses can exceed those anticipated by photopicpolarizer measurements due to additional color-balance losses. The useof dye-type polarizer for reliability reasons can result in anundesirable tradeoff situation between SCR and image brightness.

Methods for extracting heat from prior art polarization switches aremarginally successful. Fans can be used to move air over externalsurfaces, which can somewhat mitigate against performance loss concerns,and increase the power density that the product can handle beforecatastrophic failure. However, because the absorbing layer, which may betypically about 25-microns of PVA, may be buried in substantialthickness of glass which has very poor thermal conductivity, suchmeasures do not solve the thermal loading problem.

The polarizer can be thermally isolated from the subsequent elements,for example by producing two air-spaced structures. The polarizer andsubsequent functional elements of the polarization switch, for example,the liquid crystal cells and any compensation films, can be separatelylaminated between AR-coated flat-glass endcaps, with a thermal isolationstructure, such as an air-space, introduced between them. This can inprinciple substantially reduce stress birefringence while maintaining ahigh degree of optical performance. In many instances, the glass endcapsare quite thick, for example, approximately 3-12 mm, as needed tomaintain the transmitted wavefront distortion, and are composed ofborofloat glass. As such, the exit endcap of the polarizer laminate andthe entrance endcap of the subsequent assembly have the potential tointroduce their own stress-birefringence. Of particular concern is theformer, which resides directly adjacent the absorptive polarizer.

Preferred embodiments are enabling in that they; (1) substantiallyeliminate stress birefringence problems produced by polarizer thermalloading; (2) achieve a high degree of optical performance (e.g. lowtransmitted wavefront distortion); and (3) substantially eliminate thetradeoff between image brightness and stereo contrast ratio (SCR).

The function of endcaps is to provide a carrier substrate for externalantireflection coatings, which may substantially eliminate reflectionsat the input/output thermal isolation structure-glass interfaces, and toprovide the desired consistency in local integrated optical path-lengththrough the assembly. Even thin polished glass, index-matched to thepolarizer surface, can be quite effective at reducing irregularity.However, thin glass is easily distorted in the lamination process, andas such, reductions in irregularity can be accompanied by (even moreserious) introduction of optical power. This is associated withnon-uniformity in the thickness of the optical adhesive enabled by thecompliance of the substrate. When polarization-switch aperture sizes arerequired to be large, substrates in the approximately 3-12 mm range areoften used to ensure that no bending occurs in the lamination processthat can otherwise produce optical power.

Liquid crystal polarization switches used for 3D projection haveprescribed retardation values that represent a lock-and-key relationshipbetween it and the eyewear. Any retardation introduced via thermalloading disrupts that relationship, resulting in a ghost image. Theamount of retardation induced in a substrate is proportional to fourimportant parameters; the temperature distribution (thermal gradient),the coefficient of thermal expansion, the stress-optic coefficient, andthe thickness. The cone of light passing through a polarization switchproduces higher power density in the center, and thus a higherconcentration of heat in the center. This heat produces a straindistribution that for most glasses results in significant retardation.The poor thermal conductivity of glass, the non-uniform absorption ofover half the luminous output of the projector, and the substantialthicknesses typically required to maintain flatness, are all problematicfor keeping optical retardation under control for conventional glasses.Typical glass used for optical substrates has a CTE in the approximaterange of 5-10 (×10⁻⁶/° C.), in which fused-silica has a CTE roughly anorder of magnitude smaller. Other materials may have more typical glassCTE values, but have unusually low stress-optic coefficients (2-10(×10⁻⁸ mm²/N). Frequently, these are glasses with high lead content,which can pose environmental compliance issues. Fused silica has afairly typical stress-optic coefficient (3-4×10⁻⁶/° C.), but has lowoptical absorption and low CTE, and is readily available in largepieces, so it is an appropriate substrate.

FIGS. 2A and 2B are schematic diagrams illustrating two configurationsfor high power handling polarization switches, including, but notlimited to, Zscreens. FIGS. 2A and 2B may be part of an optical systemwhich may include a polarizing beam splitter, a rotator, and areflector. This optical system may generally be discussed in commonlyowned U.S. Pat. No. 7,905,602, which is herein incorporated by referencein its entirety. FIGS. 2A and 2B include two assemblies: the inputassembly which may be responsible for creating partially/fully polarizedinput light, and the second assembly may manipulate or modulate thelinear state of polarization, either passively or actively. In bothFIGS. 2A and 2B, the two assemblies may be spaced apart from one anotherby an air gap. The air gap or other thermal isolation structure may bein the approximate range of 0.5-10 mm thick.

As illustrated in FIGS. 2A and 2B, the first assembly or the inputassembly may include a linear polarizer which may have at least an inputend cap and in some cases an output end cap before the air gap. Thesecond assembly may include two liquid crystal cells or pi-cells tomanipulate or modulate the light. In some cases, a clean-up polarizermay be located prior to both of the liquid crystal cells. Further, thesecond assembly may include an end cap located adjacent to the air gapand before the first liquid crystal cell, or before the polarizer shouldthe polarizer be included in the second assembly. Additionally, anotherend cap may be located after the second liquid crystal cell. Moreover,the first assembly may receive light from a polarizing beam splitter inwhich the light may be transmitted through the polarizing beam splitterand/or reflected from the polarizing beam splitter. In some embodiments,the polarizing beam splitter may be a MacNeille polarizing beam splitteror a wire grid polarization beam splitter.

FIGS. 2A and 2B may receive light at the input end cap before the linearpolarizer. After the light propagates through the input end cap and thelinear polarizer, the light may encounter the air gap. The next end capmay receive the light from the air gap and direct the light to one ofthe polarizer or the first liquid crystal cell. The light may then bedirected from the first liquid crystal cell to the second liquid crystalcell and then exit the last end cap. As illustrated in FIGS. 2A and 2B,there may be -C compensators located between the first liquid crystalcell and after the second liquid crystal cell.

FIG. 2A illustrates a first end cap 201, a first polarizer 205, a secondend cap 210, an air spaced gap 215, a third end cap 220, a secondpolarizer 225, a first cell 230, a first compensator 240, a secondcompensator 250, a second cell 260, a third compensator 270, and afourth end cap 280.

In FIG. 2A, the first assembly includes a first end cap 201 which may bean input flat glass substrate, which may be anti-reflective coated (ARcoated). Because this substrate is up-stream in the light path from thepolarizer, it need not maintain a low level of birefringence underthermal load. The first end cap 201 can be relatively inexpensive glassin sufficient thickness needed to maintain flatness under load. Thethickness of the end cap may be in the approximate range of 3-12 mmthick. However, the first end cap 201 may be sufficiently mechanicallystable that it remains flat and does not introduce stress birefringencein elements subsequent to the polarizer. Acceptable birefringence in theglass or end cap may be in the approximate range of 5 nm/cm. This firstend cap 201 or glass substrate is followed by a first polarizer 205which may be a high-durability linear polarizer which createssubstantially polarized light. This polarizer may be a high transmissiondye-type polarizer with a polarizing efficiency that is insufficient asa stand-alone polarizer. That is, it is anticipated that a “clean-up”polarizer is contained in the second assembly, such that the combinationhas better polarizing efficiency than is possible with a single highpolarizing efficiency dye-type polarizer. The polarizing efficiency maybe greater than 99.99% or an SCR of greater than 1000:1. Although thesecond polarizer or clean-up polarizer of FIGS. 2A and 2B may beincluded in the second assembly, in another embodiment, the secondpolarizer or clean-up polarizer of FIGS. 2A and 2B may not be a part ofthe second assembly, for example when a polarizing beam splitter isemployed in the optical system. Additionally, the second polarizer ofclean-up polarizer may not be part of the second assembly whether or nota polarizing beam splitter is employed in the optical system.

The first assembly of FIG. 2A may include the first end cap 201, thefirst polarizer 205, and the second end cap 210. In the first assemblyof FIG. 2A, the first polarizer 205 may be followed by a second end cap210 or second bulk substrate, possibly similar in form-factor to thefirst end cap 201 or input substrate, but using a different material.The second end cap 210 may be composed of a material with relatively lowcoefficient of thermal expansion (CTE), relatively low stress-opticcoefficient, or a combination of each. An exemplary material issynthetic fused-silica (SFS), which has a CTE of approximately 5-10(×10⁻⁷/° C.), and a stress-optic coefficient of approximately(3-4×10⁻⁶/° C.). When absorption occurs in the first polarizer 205,raising the local temperature, the result is relatively low substrateretardation, even when using thick substrates. Low substrate retardationmay be approximately 5 nm.

The first end cap 201, first polarizer 205, and second end cap 210 ofthe first assembly of FIG. 2A are approximately index matched together,typically with an optical adhesive. For example, the index of the endcap, first polarizer and second end cap may be within approximately 0.05of one another. Desirable properties of the adhesive, apart from meetingmechanical/durability requirements include water-white transmission,low-haze, low durometer (which may be needed to, for example, isolatethe different thermo-mechanical properties of the three substrates), anda refractive index that approximately matches the three elements of thefirst assembly.

Continuing the discussion of the embodiments of FIG. 2A, the first andsecond assemblies may be spaced apart by an air gap 215. The secondassembly of FIG. 2A may include the third end cap 220, the secondpolarizer 225, the first cell 230, the first and second compensators240, 250 respectively, the second cell 260, the third compensator 270,and the fourth end cap 280. In the second assembly, the first and secondcell 230, 260, respectively, may be liquid crystal cells, for example picells. The first, second, and third compensators 240, 250, and 270,respectively, may be -C compensators, and may each be Zeon 250 -Ccompensators. The compensators may be in the approximate range of200-300. The compensators may improve contrast, among other things, forwide field of view circumstances.

In the second assembly of FIG. 2A, the second polarizer 225 may be aclean-up polarizer which may absorb a small amount of residual lightfrom the first assembly as needed to obtain a desired degree ofpolarization. This produces a very pure linear state of polarization,but with very little thermal loading on the second assembly. Due to thelow absorption, the second polarizer 225 or the clean-up polarizer maybe a less durable iodine polarizer, which can have the desirableproperties of flat spectral response, and high internal transmission, inthe approximate range of 96-99%, enabled by iodine chemistry and the lowpolarizing efficiency tolerable when using a pair of polarizers.Moreover, the low thermal loading on the second assembly may allow thethird end cap 220 or entrance substrate to be a lower cost borofloatglass. In practice, absorption at the level of, for exampleapproximately 2% produces very little heat, and therefore very littlestress birefringence on the entrance substrate, liquid crystal glasssubstrates, and the exit substrate. In a preferred embodiment, thetemperature rise due to light absorption is sufficiently low that theentire second assembly can be built using inexpensive glass, for exampleborofloat. For example, the temperature rise may be in the approximaterange of less than 10 degrees C.

FIG. 2B may include a first assembly and a second assembly. Further, thefirst assembly of FIG. 2B may include a first end cap 203, a firstpolarizer 207, and a planarization layer 209. The first and secondassembly may be separated by an air gap 211. The second assembly of FIG.2B may include a second end cap 222, a second polarizer 227, a firstcell 232, a first compensator 242, a second compensator 252, a secondcell 262, a third compensator 272, and a third end cap 282. In differentembodiments, the first and second polarizer may be either a dyepolarizer or an iodine polarizer, or any other appropriate polarizer, orany combination thereof. In the embodiment of only a first polarizer,the first polarizer may be either a dye or an iodine polarizer, or anyother appropriate polarizer.

The first assembly of FIG. 2B may include the first end cap 203 andfirst polarizer 207. In the first assembly of FIG. 2B, the second endcap 210 or the exit substrate as illustrated in FIG. 2A, may beeliminated in the first assembly of FIG. 2B, and a thin (likely ARcoated) planarization layer 209 may replace it. The planarization layeris substantially index-matched to the polarizer substrate and mayinclude a UV cure resin that is cast onto the polarizer surface. Thislayer is typically in the approximate range of 20-100 microns inthickness, but it eliminates the air-polarizer interface, functionallyremoving irregularity without requiring a bulk substrate (and theassociated cost and thickness). This planarization layer 209 mayadditionally be polished to achieve the appropriate flatness. Thismaterial may be an acrylic with a fairly high durometer, as may beneeded to avoid scratching when it is handled or cleaned. It may furtherhave a high glass transition temperature and particular surfacechemistry as may be required to deposit a high quality AR coating. Forexample, the durometer may be approximately 90 shore A and the glasstransition temperature may be approximately 100 degrees C.

A benefit of the planarization layer 209 in FIG. 2B, may be that itreduces the substrate thickness by a ratio of 1,000×-5,000×, while stillperforming the desired optical function. This reduces the retardation bya similar factor, and also helps overcome the thermal conductivityproblem responsible for trapping heat at the polarizer. Such heat isresponsible not only for retardation, but for determining the opticaldamage threshold. When lumen density is sufficient to drive thepolarizer temperature above 90 C, even high-durability dye polarizerscan fail. By eliminating the thick exit substrate, external heatextraction methods, for example fans, can be more effective, therebyreducing the polarizer temperature when light fluxes are high. Filteredair can be pushed through the channel between the exit polarizersubstrate and the input substrate of the second assembly, whicheffectively discharges the heat accumulated at the polarizer.

There are several manufacturing methods for producing a planarizationlayer. In an exemplary method, the polarizer film may be first laminatedto the first end cap or entrance substrate using a pressure sensitiveadhesive (PSA). The polarizer may have surface treatments, hard coats,or may have surface activation using, for example plasma treatment, topromote strong adhesion to the planarization layer. A liquid resin maybe dispensed onto the polarizer and a flat casting mold is pressed intothe resin, distributing it over the polarizer surface. The casting moldmay be treated with a mold-release material to discourage bonding whencured. The casting mold may be composed of a polished glass material,which is transparent to UV radiation. After the desired resin thicknessis obtained, with the casting mold aligned approximately parallel to theinput substrate, the resin is exposed to UV radiation, curing it. Thecasting mold is released from the resin using a mechanical or thermalprocess, exposing a resin surface that is conformal to the flatness ofthe casting mold. This surface can then have additional coatingsapplied, such as AR coatings.

The second assembly illustrated in FIG. 2B may include elements thatpassively or actively manipulate the state of polarization of lightexiting the first assembly. In the example of FIGS. 2A and 2B, thesecond assembly may include at least an entrance substrate, followed bya clean-up polarizer, two liquid crystal pi-cells, and an exitsubstrate. As illustrated FIG. 2B may also include first and secondcompensators 242 and 252, respectively between the first cell 232 andthe second cell 262. Furthermore, there may be a third compensator 272located subsequent to the second cell 262 along the light path. Thecompensators may be -C compensators, for example Zeon 250 compensators.

One difference between FIGS. 2A and 2B is that the first assembly ofFIG. 2A may have a bulk SFS substrate, and the first assembly of FIG. 2Bmay have a planarization layer. These elements may all be adhesivelybonded using index-matched adhesives.

Additionally, as illustrated in FIGS. 2A, 2B, 4A, 4B, and 4C when apolarizing beam splitter is employed or is not employed in the opticalsystem, the first polarizer may be a dye polarizer, an iodine polarizer,or any other appropriate polarizer. Further, when a polarizing beamsplitter is employed or is not employed in the optical system, thesecond polarizer of FIGS. 2A and 2B may be removed when a polarizingbeam splitter is employed in the optical system.

FIG. 3 is graph illustrating the stability of the stereo-contrast-ratio(SCR) for the air-spaced polarization switch of FIG. 2A under lumenloading. FIG. 3 illustrates substantial preservation of the SCR as thelumen loading is increased, versus the case of FIG. 1 for a conventionalpolarization switch. In addition to the elements discussed previously,the assemblies may include additional films that manipulate the state ofpolarization. For example, retardation films can be used to enhance thefield of view, as described in commonly owned U.S. Pat. No. 8,638,400,which is herein incorporated by reference in its entirety. Like glasssubstrates, retardation films can similarly be susceptible to stressescaused by thermo-mechanical loading. This loading may be due to thelamination process, differential CTE, which may exist even in thermalequilibrium, and non-uniform heating due to light flux distributions. Apreferred substrate is based on COP or COC substrates, which tend to berelatively immune to such stresses, versus retardation films based on(e.g.) polycarbonate (PC).

FIGS. 4A, 4B, and 4C are schematic diagrams illustrating optical systemsthat employ the high power handling polarization switches. FIG. 4Aillustrates input image light 400 that may be received by a polarizationbeam splitter 410. The polarization beam splitter (PBS) mayalternatively be a MacNeille PBS or a wire grid PBS. The PBS 410 maytransmit light along a first light path 460 and reflect light to areflector 420 along a second light path 470. In FIG. 4A, the light onthe first light path 460 may encounter a first polarization switch 450which may be either of the embodiments previously discussed in FIGS. 2Aand 2B. Further, the light on the second light path 470 may be passed toa rotator 430 which may be a half wave plate, and then the light may bedirected to a second polarization switch 440. The second polarizationswitch 440 may also be either of the embodiments discussed in FIGS. 2Aand 2B. Generally, the first and second polarization switches may besimilar embodiments, for example, the first and second polarizationswitches of FIG. 4A, may both be the embodiment of FIG. 2B.

FIG. 4B illustrates input image light 400 that may be received by apolarization beam splitter 410. The polarization beam splitter (PBS) mayalternatively be a MacNeille PBS or a wire grid PBS. The PBS 410 maytransmit light along a first light path 460 and reflect light to areflector 420 along a second light path 470. Further, the light on thesecond light path 470 may be passed to a rotator 430 which may be a halfwave plate, and then the light may be directed to a polarization switch440. The polarization switch 440 may be either of the embodimentsdiscussed in FIGS. 2A and 2B.

FIG. 4C illustrates input image light 400 that may be received by apolarization beam splitter 410. The polarization beam splitter (PBS) mayalternatively be a MacNeille PBS or a wire grid PBS. The PBS 410 maytransmit light along a first light path 460. In FIG. 4C, the light onthe first light path 460 may encounter a first polarization switch 450which may be either of the embodiments previously discussed in FIGS. 2Aand 2B. The light reflected from the PBS 410 may be passed to a rotator430 which may be a half wave plate, and then the light may be directedto a second polarization switch 440. The second polarization switch 440may also be either of the embodiments discussed in FIGS. 2A and 2B.After the light encounters the second polarization switch, the light maybe passed to a reflector 420. Generally, the first and secondpolarization switches may be similar embodiments, for example, the firstand second polarization switches of FIG. 4A, may both be the embodimentof FIG. 2B.

FIGS. 5A and 5B are schematic diagrams illustrating two configurationsfor high power handling polarization switches, including, but notlimited to, Zscreens. FIGS. 5A and 5B may be part of an optical systemwhich may include a polarizing beam splitter, a rotator, and areflector. This optical system may generally be discussed in commonlyowned U.S. Pat. No. 7,905,602, which is herein incorporated by referencein its entirety. FIGS. 5A and 5B include two assemblies: the inputassembly which may be responsible for creating partially/fully polarizedinput light, and the second assembly may manipulate or modulate thelinear state of polarization, either passively or actively. In bothFIGS. 5A and 5B, the two assemblies may be thermally isolated from eachother by an air gap or other thermal isolation structure 511, 515 madeof non-heat conducting materials or low thermally conductive materials.These non- or low thermally conductive materials may be, for example,air, greases, oils, or other solids, fluids, or gases. Low thermalconductivity means that heat transfer occurs at a low rate acrossmaterials. The units of thermal conductivity, W/mK, are watts per squaremeter of a defined surface area for a temperature gradient of one Kelvinfor every meter in thickness. The thermal isolation structure may be inthe approximate range of 0.5-10 mm thick. Some example materials andtheir typical thermal conductivities are air (0.02 W/mK), acrylic (0.2W/mK), epoxy (0.35 W/mK), glass (1.05 W/mK), and silicone oil (0.1W/mK). In some embodiments, the thermal isolation structure 511, 515 maybe made of low thermally conductive optically clear oils, greases, oradhesives. In some embodiments, the thermal isolation structure 511, 515may be made of one or more layers of materials. The layers may be madeof air, grease, oil, adhesive, or some other material.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiment may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audiovisual systems and electrical and/or opticaldevices. Aspects of the present disclosure may be used with practicallyany apparatus related to optical and electrical devices, opticalsystems, presentation systems or any apparatus that may contain any typeof optical system. Accordingly, embodiments of the present disclosuremay be employed in optical systems, devices used in visual and/oroptical presentations, visual peripherals and so on and in a number ofcomputing environments.

It should be understood that the disclosure is not limited in itsapplication or creation to the details of the particular arrangementsshown, because the disclosure is capable of other embodiments. Moreover,aspects of the disclosure may be set forth in different combinations andarrangements to define embodiments unique in their own right. Also, theterminology used herein is for the purpose of description and not oflimitation

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

The invention claimed is:
 1. A polarization switch, comprising: a first assembly operable to receive light, comprising: a first end cap; a first polarizer located to receive light from the first end cap; a second end cap located to receive light from the first polarizer; a second assembly located to receive light from the first assembly, comprising: a third end cap; a first cell located to receive light from the third end cap; a second cell located to receive light from the first cell; a fourth end cap located to receive light from the second cell; wherein a thermal isolation structure separates the first assembly from the second assembly.
 2. The polarization switch of claim 1, wherein the first end cap is borofloat glass.
 3. The polarization switch of claim 2, wherein the first end cap comprises an anti-reflective coating.
 4. The polarization switch of claim 1, wherein the first polarizer is a dye polarizer.
 5. The polarization switch of claim 1, wherein the second assembly further comprises a second polarizer located to receive light from the third end cap, and wherein the first cell is further located to receive light from the second polarizer.
 6. The polarization switch of claim 5, wherein the second polarizer is an iodine polarizer.
 7. The polarization switch of claim 1, wherein the first cell and the second cell are liquid crystal cells.
 8. The polarization switch of claim 1, wherein the thickness of the first, second, third and fourth end caps are in the approximate range of 3-12 mm.
 9. The polarization switch of claim 1, wherein the second end cap is synthetic fused silica.
 10. The polarization switch of claim 1, wherein the second assembly further comprises a first compensator located to receive light from the first cell, a second compensator located to receive light from the first compensator, and a third compensator located to receive light from the second cell, wherein the second cell is further located to receive light from the second compensator, and wherein the fourth end cap is further located to receive light from the third compensator.
 11. The polarization switch of claim 10, wherein the first, second, and third compensators are -C compensators.
 12. The polarization switch of claim 1, wherein the thermal isolation structure is in the approximate range of 0.5-10 mm thick.
 13. The polarization switch of claim 1, wherein the thermal isolation structure is an air gap.
 14. The polarization switch of claim 1, wherein the thermal isolation structure includes at least one of an oil, a grease, or an adhesive.
 15. A high power handling polarization system, comprising: a first optical assembly operable to receive light, comprising: a first substrate operable to receive light; a first polarizer adjacent to the first substrate and operable to receive light from the first substrate; a planarization layer adjacent to the first polarizer; a second optical assembly operable to receive light from the first optical assembly, comprising: a second substrate located to receive light from the planarization layer of the first optical assembly; a first liquid crystal cell located subsequent in the light path to the second substrate; a second liquid crystal cell located to receive light from the first liquid crystal cell; a third substrate located subsequent in the light path to the second liquid crystal cell; and a thermal isolation structure located between the first optical assembly and the second optical assembly.
 16. The high power handling polarization system of claim 15, wherein the first polarizer is an iodine polarizer.
 17. The high power handling polarization system of claim 15, wherein the first and second liquid crystal cells comprise pi-cells.
 18. The high power handling polarization system of claim 15, wherein the thickness of the thermal isolation structure is in the approximate range of 0.5-10 mm.
 19. The high power handling polarization system of claim 15, wherein the planarization layer is approximately index-matched to the first polarizer surface.
 20. The high power handling polarization system of claim 15, wherein the thermal isolation structure includes at least one of an oil, a grease, or an adhesive. 